A mobile computer such as a laptop personal computer (laptop PC) or a tablet device transitions from a power-on state to a power-saving state such as a suspend state or a hibernation state to lengthen operation time by a battery. For transitioning to the hibernation state, a system context of each device and a content stored in a main memory in the power-on state (hibernation data) are stored in a disk drive such as a hard disk drive (HDD) or a solid state drive (SSD) and then electricity to most of the devices is stopped. Then to resume the power-on state, the hibernation data is restored to the original positions from the disk drive, and thus the resume time can be shortened compared with the case of starting from the power-off state.
For resumption, power is turned on at all devices. Firstly, the BIOS initializes the devices where the supplying of power has been stopped, the initialization including detection and inspection of the devices, parameter setting and the like. The devices inspect themselves if needed, and when the devices become accessible by the system, the devices send a ready notification to the BIOS. When receiving such a ready notification from every device and completing the resume process for hardware, the BIOS passes the control of the resume process to the operating system (OS). If there is a device, from which the BIOS does not receive a ready notification after the elapse of predetermined time, the resume time will be delayed because the initialization of such a device is skipped and control is passed to the OS. The BIOS cannot access the device that does not send a ready notification.